1. Field of the Invention
This invention relates to a control apparatus for a vehicular automatic transmission which has transmission trains of five forward speeds or more.
2. Description of the Related Art
Conventionally, in a control apparatus for a transmission which has a plurality of transmission trains, a plurality of shift valves are connected in series in an oil supply passage which is communicated with a manual valve. In an upshift position of an upstream shift valve, the oil is supplied to a downstream shift valve. In a downshift position of the oil-supplied shift valve, the oil is supplied to a hydraulic engaging element of a corresponding transmission train to thereby establish the transmission train. For example, in a transmission of five forward speeds, there are connected in series, from an upstream side downwards, a first-second (1-2) shift valve, a second-third (2-3) shift valve, a third-fourth (3-4) shift valve and a fourth-fifth (4-5) shift valve.
If the shift valves are connected in series as described above, all shift valves on the downstream side will be affected by an abnormality of a shift valve on an upstream side, with the result that the transmission by these shift valves cannot be effected normally. Therefore, it is necessary to provide shift valves on the upstream side with a fail-safe function. However, with five or more transmission trains, the shift valves to be provided with the fail-safe function increase in number, resulting in a complicated circuit construction for the fail-safe function.
On the other hand, a solenoid valve has been recently proposed which interposed in an oil circuit for spool control of each shift valve so that each shift valve can be shift-controlled, or controlled for changeover, by an electronic control circuit via each solenoid valve. In such an arrangement, the same number of solenoid valves are required as that of the shift valves. With the increase in the number of transmission trains, the number of solenoid valves also increases to thereby bring about a complicated hydraulic oil circuits and higher costs.